This invention relates to metal articles which are subjected to elevated temperatures during use. In particular, the invention is concerned with a method for significantly improving the elevated temperature corrosion resistance of such articles whereby more satisfactory performance and longer life for such articles can be obtained.
Elevated temperature exposure of metal articles is experienced in many situations. Metal components are subjected to such conditions, for example, in various aerospace applications and in land and marine operations such as in the case of components utilized in gas turbine engines.
In all such applications, it is important to provide some means for preventing undue corrosion of the components involved since such corrosion can materially shorten the useful life of the components. Deterioration of components can also create significant performance and safety problems.
Various alloys, including most superalloys, are characterized by a degree of corrosion resistance; however, such resistance is significantly decreased when unprotected superalloy components are exposed at the operating temperatures involved in certain systems. For that reason, such components have been provided with coatings, such as aluminide coatings, which increase the corrosion resistance at the extreme operating temperatures.
Aluminide coatings are applied by methods such as the pack cementation process. In this process, the substrate chemistry and the processing temperature exert a major influence on coating chemistry, thickness and properties. Specifically, the coatings comprise a hard, brittle outer layer and a hard, brittle multi-phase sublayer that can crack when subjected to operating conditions. This leads to poor fatigue properties and the cracks can also materially reduce the corrosion resistance of the coated components.
Amother class of coatings is the MCrAlY overlay coatings where M stands for a transition metal element such as Fe, Co, or Ni. Presently, these coatings are applied by vacuum vapor deposition of MCrAlY alloy on a superalloy surface. Such MCrAlY coatings have been shown to have an advantage over aluminide coatings in providing extended life to turbine components. Specifically, MCrAlY coatings demonstrate greater corrosion resistance than aluminide coatings and exhibit superior ductility.
In the past, efforts to improve the corrosion resistance of MCrAlY coatings generally have met with only limited success. One approach is to increase corrosion resistance by either raising the amount of aluminum or adding other beneficial constituents to the coating. Unfortunately, this practice can result in embrittled coating compositions which fail prematurely under thermal and mechanical stresses. A second approach is to increase the thickness of the coating without changing the composition. Unfortunately, thicker coatings can be expensive to produce using slow deposition rate processes such as vapor deposition, and distinct thickness limitations exist because very thick coatings tend to crack or spall when subjected to the gas turbine engine environment.
Applicants' copending application Ser. No. 821,546 filed on Aug. 3, 1977, and entitled "Process for Producing Elevated Temperature Corrosion Resistant Articles", describes a process wherein coatings are applied to superalloy substrates and the like to improve the elevated temperature performance of the articles. This process involves the application of an overlay of an MCrAlY composition followed by the application of an aluminide layer. Furthermore, the coated articles are subjected to a hot isostatic pressing operation wherein pressures in excess of about 10,000 psi are applied at temperatures in excess of about 2000.degree. F.